Polyimide coatings and films have been selected for use in a number of different product applications where thermal stability and good electrical and mechanical properties are deemed necessary and/or desirable. Polyimide coatings and films having the additional property of good transparency have been used extensively as oriented films in liquid crystal display devices, fiber optic cable coatings, waveguides and protective coatings for solar cells. Although such polyimide coatings and films have good transparency, they are often colored yellow or brown as a result of a severe heat history leading to their formation. This coloration is deemed unacceptable in applications such as liquid crystal oriented films where the coloration darkens the visual field thereby impairing the function of the display device on which the film is used.
In response to this noted deficiency, various polyimide coatings and films have been developed that demonstrate a small degree of coloration and high transparency. The development of such prior art coatings and films has been guided by a series of studies that has focused on the cause of coloration in transparent polyimide films. These studies have reported that the coloration of a polyimide depends greatly upon the types of aromatic tetracarboxylic acid dianhydrides and diamino compounds selected for use as starting materials for the polyimide. In particular, these studies have reported that an aromatic diamine having the amino groups at the m-position is especially effective as the diamino compound, and the combination of it with a biphenyltetracarboxylic acid dianhydride can lead to the formation of a colorless, transparent polyimide (see Cols. 1-2, lines 64-6 of U.S. Pat. No. 4,876,330 to Higashi et al.).
The development of such prior art coatings and films has also been guided by the well-known principle that higher formation or polymerization temperatures adversely impact upon the degree of coloration of the resulting polyimide. In practice, these prior art polyimides are prepared by processes whereby the aromatic tetracarboxylic acid dianhydride and diamino compound are polymerized at temperatures of 80.degree. C. or less to form a polyamic acid solution and then, the polyamic acid is imidized by either thermal or chemical means (see Col. 8, lines 25-39 of U.S. Pat. No. 4,876,330 to Higashi et al.).
In accordance with the above, an object of the present invention is to provide a novel, essentially colorless, transparent polyamic acid solution and polyimide coating or film that serve to negate commonly held principles and beliefs.
It is a further object of the present invention to provide a process for preparing such coatings or films that also runs contrary to established findings.